1. Field of the Invention
The invention relates to a pressure-cutoff arrangement for an auxiliary power steering system in the end positions of the steering lock.
2. Description of Related Art
Pressure-cutoff arrangements of this type are known from the prior art. Thus, for example, EP 0321756 A2 indicates a recirculating ball steering system in which the working piston carries two seat valves which can be actuated in the axial direction and which strike against the housing in the end positions of the admissible steering lock and, in doing so, reduce the working pressure occurring in the working space which has been acted upon by pressure. As a result of this, the auxiliary power assistance is eliminated when this impingement occurs, so that further turning of the steering system is not possible.
A generic auxiliary power steering system, in which cutoff valves are likewise associated with the working piston, is also known from Spanish Patent Application P9202046. In the extreme positions, the cutoff valves impinge on a stop in the form of a pin which is pretensioned in the direction of the valves by a spring. The pin can be locked in the radial direction with the aid of a clamping screw. In order to set the opening point of the cutoff valves, and thereby set the stop limits, it is necessary, in this steering system, to undo the clamping screw and to actuate the steering system manually, with the wheels relieved of load, until the maximum admissible turning angle is reached. As a result of this, the cutoff valve will press the spring-loaded pin inwards into the required position. The pin can then be locked in this position and is thereby permanently set.
Finally, the nearest prior art is known from DE 4127610, in which stop means in the form of a bolt or bush form the end stops of the cutoff valves. The pistons or bushes are seated, with a press fit, on ribs constructed in the peripheral direction and can be pressed, when the front wheels are relieved of load for the purpose of setting the steering system manually, that is to say without auxiliary power assistance, into the required position, in which they are fixed automatically because of the press fit. In this form of embodiment, manual fixing, such as is known from Spanish Patent Application P9202046, is not necessary. However, the press fit of the piston or bush on the one hand necessitates very precise manufacture of the corresponding structural elements, and on the other hand permits, at most, very few repeated setting operations, since the fit becomes loose in the event of repeated changing and the retaining force falls below the opening force of the cutoff valve.